1. Field of Invention
This invention relates to fluid power devices commonly known as pumps, compressors, and fluid motors.
2. General Description of Prior Art
Persons involved in the design and production of many types of powered equipment, both portable and stationary, are becoming increasingly aware of the advantages of variable displacement in regard to control of fluid power systems. Various combinations of fluid power devices generally involving some combination of pump and fluid motor devices where one or more such devices is capable of varible displacement can provide a exceptionally versatile means of rotary power transmission popularly known as hydrostatic transmissions. The relatively high initial cost, the reliance on oil as a working fluid, and some doubts in regard to the reliability and maintenance of such transmissions has precluded their more widespread use however.
Variable displacement pumps and fluid motor devices produced to date tend to be highly dependent on lubrication. In as much as it is difficult or impossible in most prior art to segregate a suitable lubricant from the working fluid, it is not surprising that the working fluid itself is commonly relied upon to provide the necessary lubrication. The use of a petroleum based oil, often of a very specific type and weight, is therefore mandatory in most variable displacement fluid power devices commonly available.
While oils have other properties making them an excellent choice as a working fluid, oils also have disadvantages. Oils tend to be messy and can pose a risk of contamination, pollution, or pose a unacceptable safety hazard. Fluids of other types, such as water, slurries, fluid mixtures, or gases such as ambient air; fluids which may be readily available or required by a particular application cannot be used. Therefore some inherent advantages of variable displacement, such as volumetric flow control of liquids, process liquids for example, cannot be properly realized or exploited.
It is also likely that new applications not presently contemplated or seriously explored for lack of a suitable mechanism will be found for variable displacement devices capable of efficiently using fluids other than oil as working fluids. Compressors for example, can be made more efficient if capable of variable displacement as less adiabatic heat tends to be generated and more of the adiabatic heat which is produced can be usefully recovered when compressed air is provided as used rather than provided and stored. (Most compressors pump to pressures at least twenty-five percent higher than the working pressure of the compressed air system they serve and adiabatic heat generated by compression is lost or intentionally discarded for storage purposes).
Few prior art devices capable of variable displacement can be operated for more than a few minutes if a suitable working fluid is not continuously flowing through the device. In some devices, the working fluid must not only be immediately available on start up to a pump or motor device of this type whether simply rotating or working, but must also be pressurized by a external charging pump. Failure of the charging pump or charging system can result in immediate and catastophic failure and the additional pump and fluid system adds significantly to the cost and complication of the system overall.
Other disadvantages of prior art devices capable of variable displacement generally, and regardless of type, include a high cost to produce and maintain and a dependence on very clean, continuously filtered working fluids, regardless of the fluid used.
It is common in prior art devices generally regarded as rotary devices to provide a piston surface area considerably larger than the maximum piston stroke. In a fluid power device where volumetric displacement is regulated by changing the effective length of the piston stroke, (or stroke equivalent), a device having a short maximum stroke is at a considerable disadvantage. Even relatively minor internal fluid leakage or slip can significantly affect or even nullify performance at low displacement settings, particularly as fluid system pressure is increased. Accurate control of the displaced fluid flow volume in such devices can therefore be difficult to achieve and maintain.
Description of Prior Art
Heretofore several prior art devices have been proposed wherein both members of a plurality of mating piston and cylinder sets are cooperatively rotated. Most such devices rely on some type of forceful rubbing or sliding contact between members to maintain radial alignment between the members of each piston and cylinder set as cooperative rotation occurs. Forceful sliding contact between a member providing radial support for one or the other member of each piston and cylinder set is also commonly relied upon as a mechanism for ensuring cooperative rotation of the piston and cylinders members and thereby ensuring constant radial alignment of the piston and cylinder members as the members are cooperative rotated.
Irrespective of whether the piston or cylinder members of each piston and cylinder set of a given prior art device are individually and slidingly relocated as cooperative rotation occurs during operation, the members being transversely displaced by sliding tend to be relatively massive and the sliding transverse relocation must reverse direction twice each revolution of the piston and cylinder members.
There would seem to be few if any advantages gained by substitution of a lateral or transverse reciprocating inertia for reciprocating inertia along the line of piston displacement, particularly when an additional friction load as a result of forceful rubbing contact with the member radially supporting the transversely reciprocating member is considered.
In addition, and depending on the relative location of the sliding contact surfaces supporting the transversely reciprocating members, centrifugal forces as well as the reactive forces resulting from the fluid pressures developed by the device can add significantly in regard to the reciprocating sliding friction load as operating speeds are increased.
If the transversely reciprocating members are supported in a manner which takes advantage of the centrifugal forces generated to ease the sliding friction load, the effectiveness of the coupling mechanism relied on to ensure cooperative rotation of the pistons and cylinders as required to maintain radial alignment between mating pistons and cylinders during operation is proportionally diminished as operating speeds increase.
Another common disadvantage of prior art using radially aligned pistons is lack of dynamic balance. The angular spacing between members required to reciprocate transversely must change constantly during operation thereby imposing the combined radial load of the transversely displaced members asymmetrically upon a supporting rotating member, a medial member or cooperatively rotated housing, for example. The imbalance is readily apparent using a suitable end view of a prior art device of the general type noted and comparing the angular spacing between various piston and cylinder members in regard to the rotation axis of the member radially supporting each during operation.
Objects and Advantages of the Present Invention
Accordingly it is an object and advantage of the present invention to provide a fluid power device suitable for use as a pump, compressor, fluid motor, or fluid metering device capable of accurate variable displacement control while readily adaptable to the use of working fluids of disparate properties, such as oil, water, and air.
It is a further object and advantage of the present invention that most common fluids to include gaseous fluids can be accomodated over a wide range of operating speeds without vibration or pulsing, with minimal risk of cavitation and minimal headspace at maximum displacement. Friction loads are significantly reduced by replacing sliding friction with rolling friction and the rolling friction load tends to be less affected by operating speeds and fluid system pressures while significantly less dependent on lubrication.
It is a further object and advantage of the present invention that reciprocating inertia is essentially eliminated and dynamic balance can be provided at all operating speeds.
Other objects and advantages of the present invention include: In a variable displacement device according to the present invention the working fluid flow direction, rate, and pressure can be continuously adjusted whether operating or stopped from maximum to zero to maximum as desired by the device operator. When operated at constant speed, the rate and direction of working fluid flow can be rapidly changed or consistently maintained at selected rates indefinitely, to include zero displacement or null mode. Operation in null mode essentially involves only rotation without load and is virtually frictionless, and because the angular inertia of a device operating in null mode is identical to one operating in working mode, the change from null mode to working mode in either direction of working fluid flow, or vice versa, can be made very rapidly.
It is a further object and advantage of the present invention that a device according to the present invention can comprise only a few simple shapes, each easily produced with common machine tools. The simplicity of member shapes permitting fabrication of the relatively few members required from a wide range of materials to include plastics, cast metals or metal shapes, high alloy steels, ceramics, and others. Devices according to the present invention therefore tend to be reliable and are easily maintained and repaired. Few precise fabrication or repair procedures are required even in special application devices and these procedures generally involve piston and bearing fits and the like; procedures well defined and understood by most mechanics even if relatively unskilled in the production or maintenance of fluid devices generally.